In my humble opinion, a J24 or a Colgate 26 are excellent trainers (the later of which is used by the Naval Academy to teach midshipmen who've never been in a dinghy sailboat).

I certainly understand what you are saying and I'm not trying to imply that someone can't learn to be an excellent sailor without dinghy experience. I started sailing dinghies when I was 5 years old and sailed and raced everything from 9' Mintos to Thistles and Lightnings. However, there are a lot of sailors who have never sailed a dinghy who are much more accomplished sailors than I.

I spent a lot of time sailing (and some racing) in San Juan 21s and San Juan 24s. FWIW, I think there is a huge difference between a 21 and 24 foot boat. So, while a J24 would seem like a dinghy compared to either of our current boats, a smaller boat such as a Cal 20, Catalina 22, or San Juan 21 would be better I would think. I'm sure there are other boats in the 20 foot range that I'm not aware of as well.

Still, nothing beats the experience gained (or the ab work) of sailing a dinghy on a day with variable wind for really internalizing the concepts of wind, waves, weight, and water.